1. Field of the Invention
The present invention relates to a sector drive unit for a camera for driving, for example, a diaphragm device of a camera or a light-shielding device for a digital camera.
2. Description of the Related Art
In a diaphragm device for a normal silver-film-type camera (hereinafter referred to as “film-type camera”) or in a light-shielding device for protecting an image pickup device such as a CCD or a C-MOS of a digital camera, a mechanism for opening and closing an aperture by pivotal movement of sectors is employed. In both of such a diaphragm device or a light-shielding device, the size of a diaphragm opening can be adjusted by pivoting the sectors by a drive motor, or the aperture can be opened or closed. The sectors are driven by operating the drive motor so that it outputs a sufficient amount of rotation for opening and closing the aperture by the sectors. Therefore, the drive motor is driven by an electric current having a number of drive pulses corresponding to the amount of movement of the sectors. For example, in the case of the light-shielding device for digital cameras, the sectors are closed for protecting the image pickup device when not taking a picture, and a light-shielding curtain is retracted from the aperture to open the aperture when taking a picture, and then the light-shielding curtain is moved over the aperture again to close the aperture after having taken the picture. However, during such operations, the drive motor is always supplied with electricity.
As described above, it is necessary to provide a complex and expensive pulse drive circuit for applying the desired number of drive pulses of electric current for opening and closing the sectors. In addition, driving by a plurality of drive pulses generates a time-lag, which causes failure of releasing the shutter at the right moment. In the case of the light-shielding device for digital cameras, electricity is continuously supplied for retaining the sectors in a fully opened state even during image reading by the image pickup device. This results in an increase in the amount of power consumption, and generation of noise. The above-described sectors are retained magnetically at a closed position when not taking a picture. However, when the camera is subjected to an impact, for example, due to dropping or the like, there may be a case where this retained state is released and the sectors are accidently moved to the opened state.